1. Field of the Invention
The present invention relates generally to electrolyzer systems, and more specifically to a water electrolyzer used as a hydrogen generator having an engineered safety feature that prevents leaked hydrogen from accumulating in dangerous amounts within a container enclosing the electrolyzer.
2. Description of Related Art
The recovery of hydrogen and oxygen gas by means of electrolysis of water has been practiced for over a century. More recently, water electrolyzers have been used for the purpose of generating and supplying a small stream of hydrogen and oxygen gas as a fuel supplement to internal combustion engines. The hydrogen/oxygen gas stream is usually only a fraction of a percent of the intake combustion air flow but evidence has been presented to show that this small stream can reduce emissions of particulates from diesel engines and in some cases also reduce emissions of NOx and provide small increases in engine fuel efficiency. Typically these water electrolyzers are powered by electrical current from the vehicle battery or alternator and are fitted to the back of the cab compartment of a truck or under the hood of a car.
To further improve the effectiveness of these devices it is necessary to minimize their volume and mass and maximize their energy efficiency for a given intended hydrogen or oxygen output. Furthermore it is important to operate an electrolyzer that can function reliably for thousands of hours in extremes of temperature, and in the presence of continuous shock and vibration and road grime and grit. Under such conditions, it is inevitable that mass production and deployment of water electrolyzers as hydrogen generators in consumer or commercial vehicles will result in some percentage of failures that involve hydrogen gas leaking from the generator to another area of the vehicle.
Principles of Electrolyzers and of Electrolysis
The science and engineering principles behind the design and operation of water electrolyzers are well known and understood. Some general principles follow.
Electrolysis of water is the decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an electric current being passed through the water. An electrical power source is connected to two electrodes, or two plates (typically made from some inert metal such as platinum or stainless steel), which are placed in the water. Hydrogen will appear at the cathode (the negatively charged electrode, where electrons are transferred to water molecules), and oxygen will appear at the anode (the positively charged electrode where electrons are transferred from water molecules to the electrode). The generated amount of hydrogen is twice the amount of oxygen, and both are proportional to the total electrical charge transmitted through the water. Electrons carry the current in the circuit external to the electrolysis cell and in the electrodes, while charged ions carry electric current through the water or electrolyte solution.
In the water at the negatively charged cathode, a reduction reaction takes place, with electrons (e−) from the cathode being given to water molecules to form hydrogen gas:Cathode (reduction): 2H2O(l)+2e−→H2(g)+2OH−(aq)
At the positively charged anode, an oxidation reaction occurs, where water is oxidized to generate oxygen gas and giving electrons to the anode.Anode (oxidation): H2O(l)→½O2(g)+2H+(aq)+2e−
Combining these two reactions withH2O(l)→2H+(aq)+2OH−(aq)yields the overall decomposition of water into oxygen and hydrogen:Overall Reaction: 2H2O(l)→2H2(g)+O2(g)
For every two electrons the number of hydrogen molecules produced is twice the number of oxygen molecules. Assuming equal temperature and pressure for both gases, the produced hydrogen gas has therefore twice the volume of the produced oxygen gas.
In acid solution the reactions and standard electrode potentials are2H+(aq)+2e−→H2(g) E0=0.00V½O2(g)+2H+(aq)+2e−→H2O(l) E0=1.23Vgiving a Standard EMF of 1.23 V.
In base solution the reactions and standard electrode potentials are2H2O(l)+2e−→H2(g)+2OH−(aq) E0=−0.83V½O2(g)+2H2O(l)+2e−→2OH−(aq) E0=0.40Vgiving a Standard EMF of 1.23 V.
Electric current is carried through the electrolyte solution by way of movement of ions such as H+(aq) or OH−(aq). However in pure water these ions are in very low concentration so an additional electrolyte must be added to allow practical values of current to flow. Typically an alkalis such as Sodium Hydroxide (NaOH) or Potassium Hydroxide (KOH) is added in quite high concentrations. A typical value for KOH would be about 30 wt %, the concentration at which the electrical conductivity reaches a maximum.
Faraday's Law provides the relationship between the current and the rate of electrolysis,
where N the number of moles of gas released by a current I in time t is given byN=I*t/(n*F)  (1)
N is the number of electrons required to deliver one mole of gas, for hydrogen n=2 for oxygen n=4. Thus the rate of hydrogen production is given byΔI/Δt=I/(2*F)  (2)
The minimum voltage required to electrolyze water is 1.23 V but higher voltages must be applied in order to increase the current. Voltage drops occur at the electrodes due to overpotential and across the electrolyte gap between to two electrodes.
Overpotential (η) refers to the difference between the applied potential necessary to produce a current i and the equilibrium potential E0 at zero current,η=E−E0  (3)
For the anode where oxygen is produced the overpotential is related to the current density byi=i0 exp(−bη)  (4)whereb=αnF/RT and i=I/A  (5)
and where i0 is a constant relating to the particular electrode reaction and the surface on which it occurs, e.g. platinum in KOH, α is a constant usually with a value of 0.5, F is the Faraday constant, R the gas constant and T temperature in K, A is the active area of the electrode and I is the actual measured current. A similar equation exists for the cathode but with different values of i0 and α. These equations can also be expressed in terms of the overvoltage asη=B ln i0−B ln i=B ln i0/I  (6)where B=1/b.
The gap between to two electrodes is filled with conducting electrolyte but does incur a potential drop. This potential drop is given byVelectrolyte=I*R  (7)
where I is the current and R the resistance of the electrolyte, and whereR=A/d*κ  (8)
where A is the effective electrode area and d the electrode separation, κ is the conductivity of the electrolyte which is a function of the electrolyte composition and concentration and temperature.
The Current Voltage Characteristic for a single cell is therefore given byV=E0+ηanode−ηcathode+IR  (9)V=E0+Banode ln i0/i−Bcathode ln i0/i+IR  (10)
It can be seen that for a given current the voltage can be reduced by increasing the effective surface area of the electrodes, reducing the electrode separation, increasing the concentration and temperature of the electrolyte and by catalyzing the electrodes which has the effect of increasing the value of i0.
The maximum efficiency of an Electrolyzer ε, is given by,ε=ΔH0/ΔG0 
where ΔG0 and ΔH0 are the standard Cibbs Energy Change and standard Enthalpy change for the electrolysis reaction. For water electrolysis the maximum efficiency is 120%. This is greater than 100% because in principle the reaction can extract heat from the surroundings. In practice, however, the efficiency is below 100% because the driving voltage is always greater than 1.23 V.
The actual efficiency is given byε=ΔH0/(nFV)where V is the cell operating voltage at a given current. V is given by equation (10). At a practical current density of about 2 A cm−2, the cell voltage is about 2 V, and this would give an efficiency of about 74%.
Electrolyzer Design
From the above descriptions and equations it can be shown that the most energy efficient electrolyzer is one that minimizes the overall cell impedance. For an electrolyzer supplied with current from a vehicle alternator operating at a constant voltage of approximately 13 V and with a current draw limited to 20 to 30 A, the impedance of the electrolyzer is minimized by reducing the electrode gap, increasing the electrolyte conductivity and increasing the number of cells in series, usually to six. Under these circumstances the electrode area is optimized to reduce cell impedance but to remain within the chosen current draw from the alternator.
If the electrolyte is potassium hydroxide the maximum conductivity occurs at about 28% by weight potassium hydroxide. An electrolyzer with 6 cells in series with stainless steel electrodes of 200 cm2 and a spacing of 1 cm immersed in 28% KOH will operate at 12 V and about 30 A and produce about 1.3 L/min of hydrogen. This electrolyzer would require a minimum volume of about 1.5 L of electrolyte or about 1 L of water. This amount of water would be consumed in about 16.5 hours.
Inherent Risk
As the water is consumed, hydrogen gas is continually generated according to the Overall Reaction shown above. Given the size of the H2 molecule, leakage of the hydrogen gas from the electrolyzer system, no matter how well designed, is inevitable to some degree. If the leaked hydrogen is allowed to accumulate in an enclosed volume containing the electrolyzer, such as the trunk of a car, the volume of leaked hydrogen may reach dangerous volatile levels and pose an unacceptable risk of explosion, which could lead to passenger injury, defenestration, or worse.
The purpose of this invention is to minimize the risk by means of engineered safety features that makes the electrolyzer intrinsically safe. These features should be highly reliable, fail-safe, and should prevent hydrogen gas from accumulating in dangerous levels under the harshest expected operating conditions such as extreme temperatures, vibrations, and leakage.